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MSE 3143 Ceramic Materials

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MSE 3143 Ceramic Materials MSE 3143 Ceramic Materials Crystal Structures of Ceramics Assoc.Prof. Dr. Emre YALAMAÇ Res.Asst. B.Şölen AKDEMİR 2017-2018 Fall 1 OUTLINE ATOMS, ELECTRONIC STRUCTURE OF ATOMS, INTERATOMIC BONDS THE CRYSTAL STRUCTURES AND PROPERTIES OF CERAMICS 2 ATOMS, ELECTRONIC STRUCTURE OF ATOMS, INTERATOMIC BONDS The Atom Energy Levels, Orbital and Quantum Numbers Electron distribution and Orbital Diagram Ionic Diameter Interatomic Bonds Electronegativity and Polar Bonds 3 THE ATOM 4 ENERGY LEVELS, ORBITAL AND QUANTUM NUMBERS Energy levels (shell): A shell specifies the energy levels of the atom and defines as n Lower energy levels (subshell): Atomic energy levels are composed of lower energy levels or subshells. It is defined as orbitals and every subshells has specific notations, n and l l : 0 s l : 1 p l : 2 d l : 3 f Orbital: An orbital is specified by n, l, and ml, and can contain a maximum of two electrons with opposite spins Carter, C.B.; Norton, M.G.; ’’Ceramic Materials: Science and Engineering’’, Springer, 2007 5 ENERGY LEVELS, ORBITAL AND QUANTUM NUMBERS Quantum Numbers Four quantum numbers are necessary to specify the state of any electron: 1. n principal quantum number 2. l orbital shape, or orbital angular momentum, quantum number 3. ml orbital orientation, or orbital magnetic, quantum number 4. ms spin, or spin magnetic, quantum number Carter, C.B.; Norton, M.G.; ’’Ceramic Materials: Science and Engineering’’, Springer, 2007 6 ENERGY LEVELS, ORBITAL AND QUANTUM NUMBERS Quantum Numbers Are these numbers important for ceramics ? The answer, of course, is YES. The color of a ceramic, such as ruby, derives directly from transitions between energy levels. The energy levels are the result of which orbitals are occupied and their relative energies. We use transitions for chemical analysis of ceramics—certain transitions are allowed (quantum mechanical selection rules). Magnetism relates directly to the spin of the electrons. If we have more spins up than down then we have magnetization. Atomic arrangements in covalently bonded ceramics can be understood by considering hybridization of atomic orbitals. It is the sp3 hybridization of atomic orbitals in carbon that allows the tetrahedral arrangement of atoms in diamond. The s and the p in sp3 refer to the atomic orbitals. 7 ELECTRON DISTRIBUTION AND ORBITAL DIAGRAM • Electrons are settled at orbitals according to lowering the energy of atom • These electrons are located in compliance with Pauli exclusion principle: No two electrons in an atom can have the same set of four quantum numbers # Electron distribution is shown as: nl Example: 1s1 and 1s2 8 ELECTRON DISTRIBUTION AND ORBITAL DIAGRAM Orbital Diagram electron spin and the lower shell group IONIC DIAMETER • Cations are smaller than the atoms which they form • For isoelectronic cations; the higher the ionic charge the smaller the ionic radius • Anions are bigger than the atoms which they are formed. For isoelectronic anions; the higher the ionic charge the greater the ionic radius 10 INTERATOMIC BONDS • Interatomic bonds form the internal structure by holding atoms together. • Most properties of materials are highly depend on their internal structures. • Elastic modulus Strong • Thermal expansion • Strength bond • Melting point • Interatomic bonds are originated from electrostatic attraction or coulomb forces between oppositely charged particles. Net For neutral # of # of electrical atoms electrons protons charge 0 11 INTERATOMIC BONDS • Elements with unfilled electron shells are not as stable interact with other atoms in a controlled fashion such that electrons are shared or exchanged between these atoms to achieve stable full outer shells. • In discussing ceramics, we usually think of the material in terms of ions; ions with the same sign always repel one another due to the Coulomb force. • Atoms have specific potential energies individually. This potential energy decreases during bond formation, reaches minimum at equilibrium condition. Therefore stable structure forms. 12 INTERATOMIC BONDS We can divide interatomic bonds into two categories: Primary Secondary (strong) bonds (weak) bonds Carter, C.B.; Norton, M.G.; ’’Ceramic Materials: Science and Engineering’’, Springer, 2007 13 INTERATOMIC BONDS Three Chemical Bond Model 14 INTERATOMIC BONDS Ionic Bonding Energy: The importance of lattice energy • Some exothermic reactions surpass some endothermic steps with released energy from formation of Li(s) + F2(g) → LiF(s) • This reaction of energy is the strong attraction of oppositely charged ions each other. + - ° Li (g) + F (g) → LiF(g) ∆퐻 = −755 푘퐽 • Naturally LiF gas molecules could not exist. Because there has to be excessive amount of released energy to form crystal structure by combination of gas ions. + - ∆퐻°= −1050 푘퐽 Li (g) + F (g) → LiF(s) • This energy is called as lattice energy of LiF. ° Lattice energy (∆퐻) is the enthalpy change formed during decomposition of 1 mol of ionic solid to its gas ions. It effects some properties like ionic interaction strength, boiling point, hardness and solubility etc. 15 INTERATOMIC BONDS Bond Exchange Between Periodic Elements • All binary ionic compounds contain metal and non-metal. However not all metals could form binary ionic compounds with all non-metals. The bond model between berilium (Be) and Ex.: chloride (Cl) is more similar to covalent bonding rather than ionic bonding. 16 INTERATOMIC BONDS Bond Exchange Between Periodic Elements Ionic and Covalent Bonding Compounds • In most ceramic materials ionic and covalent bonding exist together. CaSO • Composed of both ionic and covalent bonds. 4 Atomic (gypsum) Number of 16 (Having 6 electrons at its outer shell) Sulfide Bonds ionically by giving 2 electrons to the structure While S O Ca Bonds covalently 17 ELECTRONEGATIVITY AND POLAR BONDS • Until now ionic and covalent bonding formed by shared or exchanged electrons is mentioned. • In reality, bonding type of materials exist between these two. • Substantial amount of compounds have polar covalent bonding which means partially ionic and partially covalent. 18 ELECTRONEGATIVITY AND POLAR BONDS Electronegativity Electronegativity is a measure of the strength with which an atom in a molecule attracts electrons. It is the contribution of electrostatic charge. As shown in the above calculation, F atom attracts shared electrons more than H atom. It means that floride is more electronegative than hydrogen. Therefore while the end of the F atom charges negatively, the end of the H atom charges positively. These partial charges cause increase in the bonding energy. 19 ELECTRONEGATIVITY AND POLAR BONDS Electronegativity Carter, C.B.; Norton, M.G.; ’’Ceramic Materials: Science and Engineering’’, Springer, 2007 20 Crystal Structure Ceramic Structures and Stable Ionic Radius THE CRYSTAL STRUCTURES Cordination Numbers ve Types AND Pauling Rules PROPERTIES OF CERAMICS Polymorphism AX - Type Crystal Structures AmXp - Type Crystal Structures AmBnXp - Type Crystal Structures Silicate Structures Carbon Structures 21 CRYSTAL STRUCTURES • Atoms bond together in metals and ceramics in distinct geometric arrangements that repeat throughout the material to form a crystal structure. The crystal structure that results depends upon the type of atomic bonding, the size of the atoms (or ions), and the electrical charge of the ions. • The smallest grouping of atoms that shows the geometry of the structure (and can be stacked as repeating units to form a crystal of the structure) is referred to as the unit cell. • Unit cell is defined as 3 different vectors having origin at the corner of a rectangular coordinate system with axial lengths and interaxial angles. 22 CRYSTAL STRUCTURES – Bravais Lattices 23 CERAMIC STRUCTURES AND STABLE IONIC RADIUS • Ceramics are composed of at least two elements (metallic and non-metallic). • Because of having more components, it has more complex crystal structures than metals. • Atomic bonding ranges from purely ionic to totally covalent (most of them exhibit combinations of two) • In ionic bonding, metal ions (cations) are charged positively. Because they give their valance electrons to negatively charged non-metallic ions (anions). 24 CERAMIC STRUCTURES AND STABLE IONIC RADIUS • Ionic radius of anions and cations (rA and rC, respectively) are critical parameters effects the crystal structure. • The metallic ions, or cations, are positively charged, because they have given up their valence electrons to the nonmetallic ions, or anions, which are negatively charged. • Because the metallic elements give up electrons when ionized, cations are ordinarily smaller than anions, and, consequently, the ratio rC/rA is less than unity. • Stable ceramic crystal structures form when those anions surrounding a cation are all in contact with that cation as shown in the figure W.D.,Callister; D.G.,Rethwisch; Material Science and Engineering, 8th Edition, John Wiley&Sons, Inc., 2011 25 CORDINATION NUMBERS VE TYPES • The coordination number (i.e., number of anion nearest neighbors for a cation) is related to the cation–anion radius ratio. • For a specific coordination number, there is a critical or minimum rC/rA ratio. This ratio may be determined from pure geometrical considerations (Tetrahedral) • The relationships between coordination number and cation–anion radii ratios are based on geometrical considerations and assuming “hard sphere” ions • The most common coordination numbers for ceramics are 4, 6 and 8 (Oktahedral) W.D.,Callister; D.G.,Rethwisch; Material Science
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